Improvement in reflecting-galvanometers



W. THOMSON. y Reflecting Galvanolpeter.

No. 209,942. Patented N0v.12 1878.

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".PEI'ES HlmUTNOGiAPMER. WASHINGTON. D. CA

- UNITED STATES PATENT OFFICE.

WILLIAM THOMSON, OF GLASGOW, SCOTLAND.

|MPRovEM ENT IN REFLEcrlNe-GLVANOMETERS.

Speciiication'forming part of Letters Patent-No. 209,942, dated November 12, 1878; application filed January 7, 1878.

To all whom it may concern:

Be it known that I, WILLIAM THOMSON, of Glasgow, Scotland, Kingdom of Great Britain and Ireland, have invented a new and useful Improvement in Beflecting-Galvanometers, of which the following is a specification My invention relates to certain improvements in the reiiecting-galvanometer for which Letters Patent of the United States No. 92.228 were granted tome on the 6th day of July, 1869, the main object of my present improvements being, rst, to so construct the instrument that a greater number of observations can be made in a given time than with the former instrument; and, second, to render the needle less sensitive to disturbing influences without making it less delicatein its action.

These objects I attain in the manner which I will now proceed toy describe, reference beinghad to the accompanying drawing, in which- Figure 1 is a vertical section of my improved reflecting-galvanometer; Fig. 2, a plan view of the same, with a portion of the casing removed and the galvonometer-coil shown in section; Figs. 3 and 4, detached views, drawn to an enlarged scale, and illustrating parts of the apparatus; Fig. 5, a part section on the line 1'. 2, Fig. ,2;A and Fig. 6, a part section on the line 3 4, Fig. 2.

A is the wooden .base of the instrument, on l which, near one end, is placed a cast or malleable ironv box or sheath', B, the latter being divided on a horizontal plane at or about mid: way between top and bottom, so as to form a lower half or base and an upper half or cover.

I In both the upper and lower halves of the box when the said coil is in position, the said spring-plates d d' being in communication with the binding-posts c c', so that the latter constitute in effect the terminals of the galvanometer-coil D. V To the centralopening of this coil D is adapted a tube, F, which is open at the rear end, but closed at the front by la glass plate, f. The front end of the tube F is threaded, for receiving a threaded ring, G, to which is adapted another threaded' ring, I, having at the inner end a glass plate, f', similar to the plate f of the tube F. Within the ring G, about midway between its opposite ends, is hung, by means of a fineflber, the mirrored needle, consisting of a` mirror, i', with four'small steel magnets secured to its rear face, as described in my former patent. (See Figs. 3 and 4.) This mirror is of such a diameter that it will hang freely, but so that it can have no material lateral vibration, within vf.

the mirror begins to be limited in one direction, and by screwing in the ring I into the ring G until the range of the mirror begins to be limited in the other direction, the chamber in which the needle hangs can be so contracted that the resistance of the air to the motion of the needle maycause a rapid extinction of the oscillations due to any sudden commencement or variation or annulment of deflecting force of the'coil upon the needles, thus securingwhat. I' terni a deat-beat action.' In other words, the limited size of the chamber prevents the oscillation of the mirrorbeyond the points ,where the linesV of` reilection reach the opposite ends of the screen.

The opening h in the front of the sheath B, through which the ray of light passes from the lamp to the mirror and is reiiected back to the screen, is formed by recessin g the faces of both the upper and lower halves of the sheath. v. I

The tube F is maintained in a central position within the coil D by means of four very small projecting knobs, m, and a spring-pin, n, by which the said knobs are pressed against the (see Fig. 3.),4j

concave sides of the external tube. The rear end of the tube is slotted ath', Fig. 2, for the reception of a suitable instrument, by which it can be turned so as to bring the point of suspension of the mirror i uppermost, when it is desired to do this, without lifting oi the upper half of the iron sheath. It can be seenwhether this adjustment is needed by looking through the tube from the rear end and, in order to permit such observation of the mirror without necessitating the raising ot' the upper halt' ot' the sheath B or the removal ot the coil D, I form in the rear portion ofthe sheath, directly in line with the tube F, an opening, s, which is closed under ordinary circumstances by a soft-iron plug, t, so that the continuity ot' the iron sheath will not be interrupted.

The upper half ot' the sheath Bis not screwed or clamped to the lower half, but merely rests upon the same; and, in order to insure the accurate adjustment of the said upper halt` to the lower half, and its maintenance in the posi tion to which it is adjusted, I form in the top of the lower half of the sheath, near one side of the same, two recesses, o and w, the former of which is conical and the latter a V-shaped slot, with its length in a direction through the center of the conical hollow.

On the under face of the upper portion of the sheath, near one side of the same, are two semi-spherical projections, fu w; and near the "opposite side of the sheath, about midway between each end,is a projection, w, with slightlycurved face. When this latter projection rests on the top of the lower half of the sheath, and the projections o' w in the recesses o and fw, the upper half of the Sheath will be accurately centered and leveled in respect to the lower half, and cannot be easily disturbed.

In front oi the sheath B is an inclined table,

`which carries two permanent magnets, L,

-pendieular to the front of the instrument.

rlhe two magnets being of equal or nearly equal strength, they will not, when exactly opposite cach other, tend to deflect the mirrored needle; but if, when the magnet M is adjusted to a position nearer to the sheath than the magnet L, there is a tendency to produce a movement of the zero toward the right, the adjustment of said magnet M to a position'farther away than the magnetLwill produce a movement ot the zero toward the left. lhese magnets therefore aord a ready' means of adjusting the zero of the instrument.

The degree of sensibility of the instrument maybe varied in the following manner, suming, as an example, that the true northseeking pole of the needle is toward the left side of the sheath and its true south pole toward the right: In order to diminish the seusibility, a powerful bar-magnet is held on the left side ot' the sheath, with its length parallel to or in the same direction as that of the needle, and with its true north pole near the iron of the sheath.

The action of the magnet while in this position will be to diminish the directive force of the needle, butafter it is withdrawn to leave a permanent increase of directive force due to magnetism induced and retained in the iron sheath of the instrument. The same may be done or the effect increased by holding the lnagnet similarly on the right side of thc sheath, with the true south pole close to the iron of the same. rlhe eli'ect is most symmetrically produced by holding simultaneously two magnets, one on each side of the case of the instrument, with their opposite poles close to the iron of the same, as described above, To increase the sensibility the directive force must be permanently diminished byperforming the same operation with the other poles of the magnets.

The needle, having been disturbed by the presence of the inducing-m agnet, will, after the magnet is withdrawn, probably assume a position of equilibrium not corresponding with its former position, thus changing' the zero of the scale. It' it is not convenient to read the delections from the new zero, it may be changed, as above described, by adjusting the sliding magnet; or the position of the zero may be changed without perceptibly altering the sensibility by holding the true north pole or the true south pole of a magnet toward the center of the front or back of the case ot' the instrument, with its axis parallel to the side of the case. i

rIhe operation of the reflecting-galvanometerhaving been described in my former patent, it will not be necessary to explain it here.

The advantages of my improved galvanometer may be summarized as follows:

First, the dead-beat arrangement of the mirror,which, by checking the oscillations ofthe spot of light and bringing it speedily to rest, enables four or five observations to be taken in the length of time hitherto required in ob taining 011e. This feature also greatly increases the usefulness of the galvanometer as a submarine or land telegraph nstrumentby rendering it comparatively free from disturbance by earth-currents.

Second, the inclosin g of the instrument in an iron box or sheath. not inclosed in an iron case is set to be very sensitive, unless it is on the astatic system, with two needles, the zero keeps constantly changing. If by a lixed magnet or magnets in the neighborhood of the instrument the directive force is made considerably less `than the horizontal component of the earths magnetic force, then the natural daily changes-of the terrestrial magnetic force, and the greater changes which take place on days of magnetic storms, make the instrument so unsteady as to be almost unmanageable. When, however, the instrument is inclosed in an iron sheath,

the needle is not sensibly influenced by varia-` When a galvanometer iron ease.

Third, the method-of supporting the upper half of the sheath, whereby accuracy in thev setting of the same and steadiness in retaining it in its place are insured.

Fourth, the movable magnets,for adjusting ,the zero of the instrument.

1. A galvanometer-instrument in which the mirrored needle is hung in a chamber for restricting its movement beyond the extreme taining the coil and its mirrored needle with lines of reection onto the screen, substantially in the manner described.

2. The combination of the mirrored needlei with the tube F and ring I, adjustable, so as to limit the swing of the needle.

3. The combination of the ring G and its mirrored needle with the adjustable glazed ring I.

4. The combination of the ring G and its mirrored needle with the adjustable tube F.

5. The combination of the ring Gr. and its mirrored needle with the-tube F, having a glazed end.

6. The combination of the galvanometer-coil, l the tube F, its four projecting knobs m, and spring-pin n.

7. The combination of the galvanometer-coilv with van inclosing box or sheath ofiron,where by the terrestrialand other external magnetism is eut oft' from the said coil.

8. The combination of the galvanometer-coil D and its inclosed mirrored needle t' with the iron box or sheath.

, 9. The combination of the mirrored needle with an iron sheath having an opening, h.

10. The combination of the galvanometercoil, having a central opening, in whichy the mirrored needle is hung, with a box or sheath having an opening, s, in line with the central opening of the coil. A

11. The combination of the iron sheath having an opening, s, with the soft-iron plug t.

l2. The sheath B, divided on a horizontal plane passing centrally through the openings h and s.

13. The combination of the upper half of Vthe sheath and its projections fu', w', and a: with the lowerhalf of the sheath and its recesses aand w.

14. The combination of the iron casing conan adjustable magnet.

15. The combination of the iron casing containing the coil and its mirrored needle with a fixed magnet, L, and adjustable magnet M.

16. The within-described method of varying the sensibility of the instrument by diminishing or increasing the magnetism lof the iron 

